This invention relates to a dual action pressure switch adapted to be provided on the higher pressure side of the cooling cycle circuit for an automobile cooling device and to operate to open the electrical switching section operatively connected to the compressor for the cooling cycle circuit and interrupt the operation of the compressor when the pressure on the higher pressure side of the cooling cycle circuit rises or reduces to an abnormal value.
A variety of dual action pressure switches have been proposed and practically employed. One of the conventional dual action pressure switches is disclosed in Japanese Laid-Open Utility Model Application No. 162534/1983. The dual action pressure switch of this type generally comprises a casing including a lower casing portion 11 and an upper casing portion 32 secured to each other, said upper casing portion having a coaxial center pressure inlet 33 and a pressure receiving chamber 36 in communication with the inlet, a diaphragm 34 pinched at the periphery thereof between the lower and upper casing portions, a piston assembly disposed on the side of the diaphragm opposite from the pressure receiving chamber and including coaxial outer and inner pistons 19, 25 which move independently of each other, a holder 30 surrounding a lower portion of the inner piston, a spring 31 normally resiliently biasing the piston assembly upwardly through the holder and allowing the assembly to resiliently move upwardly and downwardly, a pair of stationary contacts 15, 16 secured to the lower casing portion, a pair of first movable contacts 21, 22 mounted on the outer piston and adapted to engage the stationary contacts subjected to pressure acting on both the outer and inner piston and a second movable contact 28 mounted on the inner piston above the holder.
The operation of the dual action pressure switch of the Japanese utility model will be now described. When the pressure within the pressure receiving chamber 36 in communication with the pressure inlet 33 becomes a value above a first set pressure (P.sub.1) to push the piston assembly 19, 25 downwardly against the force of the spring 31, the first movable contacts 21, 22 engage the stationary contacts 15, 16, respectively. In this condition of the pressure switch, since the movable contacts 21, 22 have already engaged the second movable contact 28, the stationary contact 15, first movable contact 21, second movable contact 28 make an electric circuit to thereby turn the electrical switching section ON. When the pressure within the pressure receiving chamber 36 further rises to a value higher than a second set pressure (P.sub.2), although pressure acting on the diaphragm 34 increases, the outer piston 19 cannot move downwardly because the outer piston is engaged by the stationary contacts 15, 16, the force which biases the diaphragm 34 downwardly acts on only the inner piston 25 to push the inner piston downwardly against the force of the spring 31 to thereby separate the second movable contact 28 from the first movable contacts 21, 22 which in turn interrupts the electrical connection between the stationary contacts 15, 16 resulting in the turning of the electrical switching section OFF. However, when the pressure within the pressure receiving chamber 36 drops to a value below the second set pressure (P.sub.2), the electrical switching section turns ON immediately.
In this way, the electrical switching section repeatedly turns ON and OFF at the second set pressure (P.sub.2) to cause the so-called chattering phenomenon and produce noise and at the same time, the contacts on the electrical switching section wear away prematurely to shorten the service life of the pressure switch.